1. Field of the Invention
The present invention relates to a technique for magnetically separating and moving plate-like metal objects such as metal can lids (closures, ends) for drying, curing or other purposes.
2. Description of Related Art
Closures for metal beverage containers are generally of a circular shape with a flanged perimeter called a curl. The closures are usually made of aluminum or steel, and the curl is used in attaching the closure to a can body through a seaming operation. To aid the integrity of the seal thus formed between the can body and the closure, it is a common practice to apply a bead of sealant or adhesive ("compound") within the curl during manufacture of the closure. Different types of coatings are also selectively or generally applied to can closures and can bodies for various other purposes as well, for example, to repair damaged coatings. For the purposes of the present description, coatings, sealants and adhesives are all considered to be "liquids" applied to a workpiece.
One problem which arises in this manufacturing operation is the curing or drying of such liquids. Recently there has been increased interest in the use of water-based sealants in the container industry, which may take 3-4 days to dry to an acceptable state for application of the closure to a can body. This was not a severe problem for solvent-based liquids, because the volatile solvent quickly evaporates and is acceptably dry for application of the closure to a can body typically within 48 hours.
In the past, can closures were heated to aid the drying or curing process typically either by infrared radiation or convection heating. These systems, especially the convection heating systems, tended to be large, bulky and expensive to operate due to inefficient energy usage. The parent applications describe a system for heating can ends inductively.
Since the present invention may be useful for most, if not all, of the various operations during manufacture, including application of liquids, drying of such liquids, and even transportation of closures from one station to the next, all such operations may be referred to herein collectively as "treatments".
Metal can closures are typically conveyed through treating apparatus in either of two ways. They can be conveyed by a conveyor belt, in which case the closures lie flat on the belt or they can be stacked within a track or cage, in abutting face-to-face contact with each other ("in-stick"). The former technique is exemplified in Collins U.S. Pat. No. 4,017,704. In the latter technique the closures are pushed through the apparatus in a direction transverse to their faces. Treating of can ends being pushed through in-stick would require less floor space since many more can ends can be packed into a given length of track. The technique is not often used in heat treating apparatus because convection air currents cannot heat or dry the faces of the can ends directly.
Sullivan U.S. Pat. No. 4,333,246 attempts to address this problem in the context of convective drying techniques. In Sullivan, the workpieces are pushed through a curvilinear path defined by a constant width trackwork, allowed to pivot on the portions of the workpieces in proximity to the shorter radiuses whereby fan-like separation of the portions in proximity to the longer radius occurs. Sullivan uses this trackwork to partially separate can ends as heated air is directed toward the separated portions.
The Sullivan technique has a number of major disadvantages. First, though one portion of each of the workpieces is separated from the other workpieces, there is always another portion of the workpieces (the portions in proximity to the shorter radiuses) which are touching other workpieces. The pieces are only fanned, not truly separated. Thus, if the apparatus is being used to cure liquids applied selectively on can ends, for example, it can be used only where the selectively applied liquid has been applied somewhere other than around the circumference where the ends are likely to touch each other. Additionally, the pressure on the portions of the ends which do touch each other, caused by the forces pushing the ends along the track, can soften and/or damage the metal of the ends or their coating. Moreover, the Sullivan apparatus can generate only limited separation between the fanned portions of the can ends, since greater separation requires tighter curves in the trackwork, which in turn requires greater force and stronger materials in the equipment which pushes the ends along the track. Nor can the technique be used for long conveyance paths, for the same reason, even if the curves are kept shallow. Still further, Sullivan's technique will not work well with can ends which have pull rings, since these can ends do not nest well and are likely to scratch each other if they touch.